It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone", the presence of the marker is detected and an alarm is generated. Some markers of this type are intended to be removed at the checkout counter upon payment for the merchandise. Other types of markers remain attached to the merchandise but are deactivated upon checkout by a deactivation device which changes a magnetic characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
A known type of EAS system employs magnetomechanical markers that include an "active" magnetostrictive element, and a biasing or "control" element which is a magnet that provides a bias field. An example of this type of marker is shown in FIG. 1 and generally indicated by reference numeral 10. The marker 10 includes an active element 12, a rigid housing 14, and a biasing element 16. The components making up the marker 10 are assembled so that the magnetostrictive strip 12 rests within a recess 18 of the housing 14, and the biasing element 16 is held in the housing 14 so as to form a cover for the recess 18. The recess 18 and the magnetostrictive strip 12 are relatively sized so that the mechanical resonance of the strip 12, caused by exposure to a suitable alternating field, is not mechanically inhibited or damped by the housing 14. In addition, the biasing element 16 is positioned within the housing 14 so as not to "clamp" the active element 12.
As disclosed in U.S. Pat. No. 4,510,489, issued to Anderson, et al., the active element 12 is formed such that when the active element is exposed to a biasing magnetic field, the active element 12 has a natural resonant frequency at which the active element 12 mechanically resonates when exposed to an alternating electromagnetic field at the resonant frequency. The bias element 16, when magnetized to saturation, provides the requisite bias field for the desired resonant frequency of the active element. Conventionally, the bias element 16 is formed of a material which has "semi-hard" magnetic properties. "Semi-hard" properties are defined herein as a coercivity in the range of about 10-500 Oersted (Oe) and a remanence, after removal of a DC magnetization field which magnetizes the element substantially to saturation, of about 6 kiloGauss (kG) or higher.
In a preferred EAS system produced in accordance with the teachings of the Anderson, et al. patent, the alternating electromagnetic field is generated as a pulsed interrogation signal at the store exit. After being excited by each burst of the interrogation signal, the active element 12 undergoes a damped mechanical oscillation after each burst is over. The resulting signal radiated by the active element is detected by detecting circuitry which is synchronized with the interrogation circuit and arranged to be active during the quiet periods after bursts. EAS systems using pulsed-field interrogation signals for detection of magnetomechanical markers are sold by the assignee of this application under the brand name "ULTRA*MAX" and are in widespread use.
Deactivation of magnetomechanical markers is typically performed by degaussing the biasing element so that the resonant frequency of the magnetostrictive element is substantially shifted from the frequency of the interrogation signal. After the biasing element is degaussed, the active element does not respond to the interrogation signal so as to produce a signal having sufficient amplitude to be detected in the detection circuitry.
In conventional magnetomechanical EAS markers, the biasing element is formed from a semi-hard magnetic material designated as "SemiVac 90", available from Vacuumschmelze, Hanau, Germany. SemiVac 90 has a coercivity of around 70 to 80 Oe. It has generally been considered desirable to assure that the biasing magnet has a coercivity of at least 60 Oe to prevent inadvertent demagnetization of the bias magnet (and deactivation of the marker) due to magnetic fields that might be encountered while storing, shipping or handling the marker. The SemiVac 90 material requires application of a DC field of 450 Oe or higher to achieve 99% saturation, and an AC deactivation field of close to 200 Oe is required for 95% demagnetization.
Because of the high level required for the AC deactivation field, conventional devices for generating the AC deactivation field (such as devices marketed by the assignee of the present application under the trademarks "Rapid Pad 2" and "Speed Station") have been operated in a pulsed manner to limit power consumption and comply with regulatory limits. However, because the AC field is generated only in pulses, it is necessary to assure that the marker is in proximity to the device at the time when the deactivation field pulse is generated. Known techniques for assuring that the pulse is generated at a time when the marker is close the deactivation device include generating the pulse in response to a manual input provided by an operator of the device, or including marker detection circuitry within the deactivation device. The former technique places a burden on the operator of the deactivation device, and both techniques require provision of components that increase the cost of the deactivation device. Also, even pulsed generation of the deactivation field tends to cause heating in the coil which radiates the field, and also requires that electronic components in the device be highly rated, and therefore relatively expensive.
The difficulties in assuring that a sufficiently strong deactivation field is applied to the marker are exacerbated by the increasingly popular practice of "source tagging", i.e., securing EAS markers to goods during manufacture or during packaging of the goods at a manufacturing plant or distribution facility. In some cases, the markers may be secured to the articles of merchandise in locations which make it difficult or impossible to bring the marker into close proximity with conventional deactivation devices.